The present invention relates to WDM (Wavelength Division Multiplexing) optical transmission equipment and more particularly to the optical transmission equipment located in a relay station.
To reduce transmission system cost, improvement of the transmission rate as well as increased density and distance capability of WDM have been sought. In general, however, improvement of the transmission rate entails problems in realizing dispersion compensation over a wide band. Dispersion compensation means compensation for waveform distortion of light signals based on the dispersion of the light signal through a transmission line. A fiber optic line contains dispersion characteristic parameters that depend on the wavelength of the light signal. The dispersion effect becomes greater as the fiber is lengthened. In particular, with dispersed compensation for wide band applications, it is quite difficult to achieve optimal compensation over all the bands.
Optical transmission technology effectively employs optical amplifiers to boost the signals on a fiber. In an optical amplifier, however, noise and gain characteristics that restrict transmission also depend on wavelength. When optical amplifiers are placed at various stages along the transmission line, the number of stages between the amplifiers affects the transmission, that is, the critical distance of the transmission is variable depending on the wavelength of the light signal. In WDM transmission, therefore, systems have generally been configured to suit the most limited wavelength characteristic, that is, the wavelength that has the shortest transmission distance. This is because a light signal of any other wavelength may be reliably transmitted. For the technology of WDM, one may refer to the optical amplifier described in JP-A-8-278523 (corresponding to U.S. Pat. No. 5,675,432).
The wavelength dependency described above creates difficulties in making the transmission line longer as WDM density is increased. This tradeoff is an obstacle to configuring the transmission system at low cost.
Further, a new technology has been studied for dropping or adding a signal wavelength of part of a wavelength band at a node located on a transmission line. With application of this technology, the multiplexed light signals transmitted on the same fiber by different nodes may be transmitted as WDM light. Since the transmission condition depends on a signal wavelength, and multiple wavelengths are typically transmitted on the different bands, which are then transmitted all together, achieving efficient dispersion compensation on all bands, has proved difficult.